Ancient

I hope I can clear things up.
The bad news is that machine can't take the ethernet card you bought. The good news is that machine already has ethernet built in, albeit an ancient form called AAUI-15 (Apple Attachment Unit Interface). More found
here
In order to connect your 7100 to a modern network, you need to buy a Tranceiver to convert it to 10BaseT. They are old but can still be found on sites like
www.ebay.com.
When setting up a network on pre-OS X Macs like OS 9, the manual to your ethernet card was wrong/not for Mac.
Apple - control panels - TCP/IP - Connect via: Ethernet
Also it helps to know what OS your 7100 is running.
Apple - About this Mac
n8